1. Field of the Invention
This invention relates to highly-absorbent webs and methods of making them. More specifically it pertains to an improved process of forming such webs of chemically modified cellulose fibers that are not only highly-absorbent but have excellent tactile properties such as softness and drape. Such webs have adequate strength and controlled wicking properties making them particularly suitable as a component of products where it is desirable to draw liquids away from the surface and concentrate them in a particular area or location. Such applications include, by way of example and not limitation, disposable diapers, sanitary napkins, tampons, wipers, surgical sponges, and the like.
2. Description of the Prior Art
The chemical modification of cellulose to increase its absorbency has been previously described and can be considered in a broad sense to fall into three major classifications in terms of methods:
(a) chemical substitution, etherization or esterification;
(b) chemical substitution plus crosslinking; and
(c) polymeric grafting.
For example, U.S. Pat. No. 3,670,679 to Mitchell is directed to absorbent fibers formed by extruding solutions such as those prepared from a hydroxyalkyl cellulose. As examples of category "(a)" above, Bernardin U.S. Pat. Nos. 3,658,790 and 3,691,154 disclose absorbent fibers in batt-like mats formed from phosphorylated cellulose or its acid form and products incorporating them. An example of category "(b)" above is disclosed in U.S. Pat. No. 3,589,364 to Dean et al which discloses absorbent structures including crosslinked fibers of carboxymethyl cellulose and products made therefrom. Category "(c)" above is exemplified by the formation of acrylonitrile grafted cellulose absorbent fibers and products as disclosed in U.S. Pat. Nos. 3,194,727 to Adams et al; 3,455,643 to Gruber et al; 3,065,041 to Sven; and 3,046,078 to Salsbury.
U.S. Pat. No. 3,997,647 to Lassen, assigned to the assignee of the present invention, discloses an extrusion process for forming highly-absorbent filamentary webs from gel-like extrudates. These extrudates include swollen fibers of chemically modified cellulose which have not lost their individual identities. By extrusion the fibers are aligned and interbonded to form filaments having channels and capillaries which provide high absorbency and excellent wicking properties. However, the interfiber bonding, believed to be hydrogen bonding, produced by the method of the Lassen patent tends to produce a harsh, stiff web unless solvent drying or other special drying means are employed. Solvent drying is, of course, relatively expensive in terms of operating costs as well as capital investment required for recovery equipment. In addition, the use of solvents requires precautions to be taken against ecological, safety and health hazards.